The present invention relates generally to radar systems, and more particularly, to a three dimensional imaging radar system that is adapted to create three dimensional images of physical objects at relatively short range.
Over the past several decades, different types of radar data displays have been developed to help the radar operator interpret the information generated by a radar system. The most common of these displays is a two dimensional (top down) display known as a plan position indicator (PPI) display. The PPI display provides information about the slant range and bearing from the radar to radar targets.
In a common two dimensional air traffic control radar, for example, an antenna beam is normally shaped to create a vertical fan beam so that all airborne targets from near the ground to a maximum altitude of interest are detected as the radar sweeps a 360 degree path around the radar. This type of radar system provides only azimuth and slant range to a target but does not provide any information about the target's height above the ground. Each target is represented as an intensified dot on the radar display. By observing changes in the position of the dot from radar scan to scan, the operator can identify moving targets as well as fixed targets on the display.
Many other forms of radars and radar displays have been developed including an SPS-39/48 three dimensional radar that provides elevation scanning information to determine target altitude. The displays on these radar systems display intensified spots of azimuth versus range or elevation angle versus range to targets.
The present invention is an improvement to a previously developed three dimensional radar system manufactured by the assignee of the present invention. The previously developed system uses a 12 inch diameter, center-fed subreflector, comprising a parabolic fixed dish antenna. In this system the radar transmitter signal is generated by circuits located directly above the fixed antenna. The signals are fed directly to a small "circular horn assembly" that is located in the center of the antenna. A small subreflector mounted in front of the horn and dish, reflects the energy back to the parabolic dish antenna which then radiates the energy with the proper radar beam shape. The signal reflected from the dish antenna is beamed straight down to a flat splash plate reflector that is located directly below the dish antenna. The dish antenna has a problem wherein strong reflections coming from the subreflector prevent reception of some weak signals. The dish antenna also prevents radar operation in two radar bands at the same time.
In the prior system, the movable flat splash plate reflector is located directly below the dish and is oriented at approximately a 45 degree angle with respect thereto. The splash plate reflector is continuously rotated (360 degrees) up to two times a second. The splash plate reflector may be stepped in 0.5 degree increments every revolution. This allows the antenna beam to be positioned or scanned a full 360 degrees in azimuth and .+-.15 degrees in elevation. The splash plate reflector has the disadvantage of covering too much area and requiring a long time to perform an elevation scan. The elevation beam can only be moved once each rotation of the antenna. It has a further limitation that even if a small area is to be viewed, the scan rate is limited by the 360 degree rotation rate. The advantage is simplicity and lower cost.
In the prior system, the primary radar operates at 56 GHz. A separate 10 GHz transmitter, receiver, and antenna (ATR) assembly was built to operate at 10 GHz. That radar performed as expected, but it was desired that the system have the capability to switch between frequencies without physically removing and replacing the ATR assembly.
The prior radar is based upon linear swept frequency continuous wave (SFCW) technology, which is well known in the art. In any SFCW radar the key parameter for performance is sweep frequency linearity. The better the linearity, the better the target resolution and noise. The prior system uses an analog sweep linearity control. While the system performs quite well under normal operation, it requires almost 1/2 hour to warm up and stabilize. It is also quite difficult to set up and maintain. The system relies on delay line compensation which requires about 30 feet of very low loss coaxial cable to be wound around the back of the reflector dish.
The prior system uses a digital fast Fourier transform (FFT) circuit to convert the received signals to range cells containing the amplitude of the return signal in each range cell. The amplitude values (8 bits) from each range cell are sent to a two dimensional digital scan converter (DSC). The DSC converts antenna pointing information into two dimensional addresses (X and Y) for storage in a buffer memory where the amplitude values for each "area cell" are stored for later processing.
There is no additional signal processing or filtering in the prior system. A computer performs the conversion from two dimensional to three dimensional plotting and target threshold and filtering processes. While the system performs reasonably well, there is a limit in just how much the computer can do in real time.
The prior system uses a two dimensional buffer memory that stores target amplitude information. The hardware buffer is 320 (X) by 200 (Y) cells, requiring 64K bytes (8 bit amplitude) storage. The position encoders located on the antenna translate the antenna pointing position (azimuth only) to SIN and COS values that are multiplied by the range cell number to determine which of the 64K memory cells to place the amplitude information.
The output of this buffer is transferred to a digital signal processor (DSP) once each rotation of the splash plate reflector. The buffer is "flushed" and the information from the next rotation of the antenna is stored. As the antenna elevation position is changed, an elevation correction factor is used to correct for that change. This is a very computational intensive process for the computer.
The prior system requires direct connection of a keyboard and a VGA color display. No remote capabilities are provided.
Accordingly, it would be an advance in the radar art to have a three dimensional radar that may be switched between two operating frequencies, that improves reception of weak signals, and that has improved signal processing capabilities and processing efficiency.